Dr. Jeff Ritterman is vice president of the board of directors of the SF Bay Chapter of Physicians for Social Responsibility. He is the retired chief of cardiology at Kaiser Richmond and a former Richmond city councilmember. Follow him on Twitter @JeffRitterman.

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Fernando Manas, a biologist at the National University of Rio Cuarto in Argentina, has been investigating the effects of pesticides for years. He believes that glyphosate spraying is causing cancer by inducing DNA damage, and his research has documented genetic damage in those exposed. When Manas studied people who spray pesticide while working in the soy industry in Córdoba, Argentina, he found significantly more DNA damage in their lymphocytes than those in an unexposed group. Glyphosate was one of the most commonly used pesticides by the workers.

Genetics researchers from the Pontifical Catholic University in Quito, Ecuador evaluated Ecuadorians living in the Sucumbíos district in northern Ecuador for evidence of DNA damage. The Colombian government had heavily sprayed the Sucumbíos district with glyphosate to eradicate illegal coca crops. People exposed to the herbicide developed a number of acute symptoms, including abdominal pain, vomiting, diarrhea, fever, heart palpitations, headaches, dizziness, numbness, insomnia, depression, shortness of breath, blurred vision, burning of eyes, blisters and rash. When compared to a control group, they also showed significantly more DNA damage.

In addition to the DNA and cell division research, scientists have explored glyphosate's association with cancer in tissue culture studies. In these experiments, researchers grow cells in a small dish with nutrients and add various chemicals to test their effects.

In 2010, researchers in India exposed mouse skin cells grown in tissue culture to glyphosate. When the herbicide was added, the cells became cancerous.

Scientists in Thailand studied the impact of glyphosate on human estrogen-responsive breast cancer cells in tissue culture. Hormone-responsive breast cancer cells are known to grow when exposed to estrogen. And according to their published results in 2013, glyphosate also stimulated these cells to grow. The herbicide was able to bind to the cancer's estrogen receptors, thus mimicking the effects of estrogen and accelerating tumor growth. Scientists refer to this as "endocrine disruption." An endocrine disruptor is a chemical that can mimic or block a hormone. Because hormones work as chemical messengers at very low doses, even a minute dose of an endocrine disruptor can lead to serious illness.

Glyphosate's links to cancer have also been assessed in studies with a variety of test animals for more than three decades. One of the earliest studies was conducted from 1979 to 1981, under the auspices of the United Nations Environmental Program, the International Labor Organization, and the World Health Organization. Rats exposed to low levels of the herbicide developed testicular cancer. A larger dose did not produce the cancer. Unfortunately, at the time of the experiment, it was not understood that certain substances have more potent effects at lower doses than at higher doses, and so the evaluators erroneously dismissed the results.

In a study from the Institute of Biology at the University of Caen in France, researchers studied glyphosate's effects on rats, and found that glyphosate doubles the incidence of mammary gland tumors. These cancers also developed much faster in rats exposed to glyphosate than in controls. There was also an increase in cancers of the pituitary gland. Originally published in 2012, the report was retracted after the biotech agriculture industry complained. But after extensive review failed to show any fraud or problem with the data, the report was re-published in 2014.

Human epidemiologic studies also have shown a link between glyphosate and cancer.

Argentine physicians working in areas in which glyphosate is heavily sprayed have reported significant increases in cancer incidence. In Sante Fe province, which is an area of intensive herbicide spraying, a house-to-house epidemiological study of 65,000 people found cancer rates two to four times higher than the national average.

Two villages in Chaco province also raised concerns about glyphosate's association with cancer. Researchers compared residents of the heavily sprayed farming village of Avia Terai to people in the non-sprayed ranching village of Charadai. In the farming village, 31 percent of residents had a family member with cancer while only 3 percent of residents in the ranching village had one.

Dr. Avila Vasquez, a doctor working in the heavily sprayed region of Barrio Ituzaingo, noted that cancer was responsible for 33 percent of the deaths in the region, while the cancer death rate in the big cities was only 19 percent.

In addition, scientists from the International Agency for Research on Cancer (IARC), who have analyzed studies spanning almost three decades, have found a positive association between organo-phosphorus herbicides, like glyphosate, and non-Hodgkin's lymphoma. IARC researchers found that the B cell lymphoma sub-type is strongly associated with glyphosate exposure. As mentioned earlier, the IARC published a monograph last month classifying glyphosate as probably carcinogenic.

The most recent research raising concerns about glyphosate's connection to cancer is the linkage to lymphoma. Scientists from the Agency for Toxic Substances and Disease Registry, a branch of the US Department of Health and Human Services, who specialize in illnesses caused by toxic substances, published results of the US Atlantic Coast Childhood Brain Cancer Study in 2009. That study compared children with brain cancer in Florida, New Jersey, New York, and Pennsylvania to age-matched controls. The researchers found that if either parent had been exposed to glyphosate during the two years before the child's birth, the chances of the child developing brain cancer doubled.